There is an increasing demand for large-scale industrial curing of UV curable coatings and inks requiring high speed or ultra fast processing for improved productivity. However, at higher print speeds, problems with inconsistency in print quality and poor curing efficiency may be encountered.
In UV curing of photo-curable inks and other coating materials, UV energy is absorbed by a sensitizer and initiates a curing process, e.g. causing polymerization of monomers, which dries and hardens the ink or coating material. The rate of the curing process usually depends on many factors, such as, the type of chemical compound, the UV light wavelength and intensity, the thickness of the coating, surface conditions, dissolved oxygen levels, and other process parameters or ambient conditions.
Several competing processes contribute to the overall reaction during photo-polymerization of UV curable inks. The general process starts from light being absorbed by photo-initiators to create free radicals, which are required to initialize polymerization of monomers in the ink formulation, which causes an increase of viscosity. However, because of the high reactivity of oxygen, initially free radicals are consumed by oxygen dissolved in the ink, and/or diffused oxygen from outside, i.e. in the ambient air. The polymerization reaction dominates only after dissolved oxygen concentrations have been consumed so as to fall to a sufficiently low level, and after the system viscosity is above a certain level such that the oxygen diffusion rate is slow enough.
For high speed and ultra-fast printing, conventional approaches to increase the rate of curing, and to increase efficiency to overcome oxygen related problems, have been focused on providing higher intensity UV illumination to enable faster processing, i.e. simply increasing power input. Unfortunately, increasing power input does not necessarily solve the problems of poor or inconsistent print quality. At the same time, since UV curing is an energy intensive process, and with the increased global concern regarding energy usage and the environment, there is also a need to design more energy efficient systems, and reduce power demands, particularly for large-scale industrial applications.
In the area of UV lamp design, there have been two main approaches to increase the efficiency of UV curing systems. The first one has focused on improving the ballast efficiency, and the other one is to minimize light loss by modifying reflector design. Using both methods, UV curing systems using UV lamps manufactured, for example, by IST METZ were recently reported to provide an increase in efficiency of 40%. Compared to conventional ballasts, square-wave ballast technology, such as used in UV lamps by GEW, for example, can reduce energy consumption by up to 30% for an equivalent cure. Both approaches aim at increasing the amount of UV irradiation delivered to the UV curable materials. However, current ballast efficiency is now typically higher than 95% and most reflector designs have already been optimized to direct the maximum amount of the light to the substrate. This leaves little room for further improvement in the amount of UV irradiation with unit amount of input electrical power. Therefore, there is a pressing need for other novel approaches to improving curing efficiency for high speed processing.
UV inkjet printing technology is moving forward rapidly as it displaces traditional printing methods. For increased throughput, there is always a need for improved UV system curing efficiency, for large scale and ultra high speed curing, in industrial sectors such as digital printing, packaging, and automotive applications. For a UV curing system typically used in inkjet printing applications, the FWHM of the UV beam profile is about 2-6 cm. Such a narrow beam profile only produces an illumination of about 10-30 ms for single scan in a wide format inkjet printer with a scanning speed of about 2 m/s. Manufacturing environments do not typically provide an oxygen free environment during the curing process (in view of expense), and therefore oxygen acts as a barrier to slow down the process. An illumination time of 10-30 ms is not usually long enough for free radicals to consume oxygen because of the inherent reaction rates. This results in the need for multiple exposures of the ink to achieve full cure. The specific exposure time required is a function of the ink chemistry, which varies from supplier to supplier, but as a general rule cumulative exposure times should exceed 50-100 ms. As scanning speeds increase for higher productivity, the illumination time becomes even shorter. Such limitation requires the industry to use even larger numbers of scans to achieve acceptable curing result. This does not satisfy the current and upcoming needs for higher productivity.
In one approach to increase the cure speed, U.S. Pat. No. 3,983,039 teaches a lamp unit with a single light source and an elongated reflector producing a diffuse lower intensity region for pre-cure, to seal the surface to reduce oxygen diffusion, followed by a high intensity region for the main cure. In practice, surface curing by intermediate or low level of UV radiation is found to be less effective than use of a higher level of UV radiation. As is known, oxygen has to be consumed to a certain level before polymerization can start and oxygen consumption has high efficiency unless the light intensity reaches certain threshold intensity. Below this threshold, oxygen consumption is slower than oxygen diffusion from outside so the polymerization reaction will fail to start. In many cases, a beam of this profile, providing diffuse lower intensity radiation at the leading edge of the light source actually extends the region of light below the threshold for initiating curing, and thus wastes light and results in poor print quality. Also, for many UV curing applications in digital printing, particularly wide format inkjet painting, a very large lamp width having an extended reflector such as taught in U.S. Pat. No. 3,983,039 is not suitable because of space limitations for lamp heads in existing printers.
Alternatively, in the past decades, UV light source companies have taught the use of extremely high intensity light for fast cure. For example, U.S. Pat. No. 5,945,680 describes an apparatus with a focusing of the light to a comparatively narrow light line with a high light intensity by a rod-shaped lens. For free radical induced polymerization, there is a simple relation between the overall rate of polymerization, Rp, and the light intensity, Rp=a(I)b. The power factor, b is about 0.5, however it is smaller when the light intensity is extremely high. The landmark study by Dr. S. Jonsson, “Secrets of the Dark”, confirmed that increasing intensity 20 times increased the maximum polymerization rate by only about 50%, which indicates that using extremely high intensity to increase polymerization rate is not a very efficient way of utilizing light. In view of the non-linear relationship between light intensity and rate of polymerization, at increasingly higher intensity, in practice, less improvement in polymerization rate and degree of conversion is possible. In addition, to achieve extremely high intensity, the beam must be focused so that the optical profile in a lateral direction of such systems is narrow, allowing for only extremely short illumination time in high speed processing. Short illumination times are problematic because there is a minimum period of exposure needed to consume residual and diffused oxygen before curing proceeds. The time period is determined by the kinetics of chemical reactions for consuming oxygen. At ultra fast process speeds, such a narrow optical profile does not provide enough illumination time required to overcome oxygen inhibition, which is required to achieve good cure result.
It is well known that all UV curing processes in air have to overcome oxygen inhibition effects to achieve a satisfactory curing quality. However, with pressing requirements for higher productivity, the relative speed between the curing light source and substrate increases. This pushes the illumination time closer to the induction time, which is required as a minimum illumination time. Traditional approaches to overcoming limited processing time for high speed print, i.e. further increasing light intensity, fail to resolve the loss of curing efficiency, because illumination with a narrowly focused higher intensity light effectively makes the illumination time even shorter.
As mentioned above, there are two sources of oxygen to be consumed: the residual oxygen in the UV curable material, i.e. in the ink, and the diffused oxygen from outside. The residual oxygen in the ink can be consumed by a high intensity UV light in a reasonable short time period. However, oxygen diffusion is a dynamic process, which will slow down when the viscosity of the bulk material increases because of the chain reaction in photo-polymerization. Such chain reaction takes a certain amount of time, which is in sub-second range, to build a network in the bulk material with viscosity high enough to compete with oxygen diffusion from outside. Traditional methods of increasing light intensity for a high speed UV curing process may consume residual oxygen in the ink, but if ultrahigh speed processing is needed, and the allowed exposure time is close or even less than the induction time, such method of increasing light intensity fails to provide satisfactory curing quality. This results in low light utilization, and a low system curing efficiency.
While it has long been recognized that the oxygen inhibition effect exists, in attempting to solve the problem by simply using more power, i.e. using extremely high intensity illumination for a short duration, the industry has failed to recognize the significance of the problem associated with the kinetics of oxygen inhibition. That is, the time scale of the kinetics of oxygen inhibition is longer than the illumination time of the substrate for high speed processing using such narrow focused optical profiles. Consequently, illumination at extremely high intensity, particularly above a certain saturation level, and for shorter illumination time, leads to low efficiency of light utilization for photo-polymerization for effective UV curing. The use of higher power and higher intensity light sources also interferes with print quality on temperature sensitive substrates such as PVC, thin films and thermally activated substrates. Print quality is reduced because the energy delivered by the curing system that is not consumed by the curing process creates heat that can deform the substrates. This can lead to warping of rigid substrates on flatbed style wide format printers, or shrinkage of flexible substrates.
Since advances in wide format printing system design are driving the speed of printing higher, and it is expected that with current equipment, the curing efficiency of light delivered to the ink will continue to fall due to ever decreasing exposure times. As the curing efficiency falls, the degree to which the ink is cured for a single pass of the light source will be reduced. This will lead to inconsistent print quality when print samples are compared between slower print systems, and higher speed systems.
In attempts to overcome these problems, the digital print industry has taken two main strategies to move to higher speed printing:                1. reducing ink deposition and using very high powered lamps, and        2. increasing the number of passes of the light source to accumulate a sufficient dose of UV.        
However, reducing ink deposition limits the print quality. By increasing the number of passes, it slows the printing process down, because each pass requires time. As dark curing plays an important part in the chemical reaction, the time period between each illumination, which varies from printer to printer, may cause inconsistencies in print quality. In addition, for high coverage printing, the ink adhesive and potential surface finish will be a function of the number of passes—leading to potential print quality inconsistencies from different models of printers, or from the same printer if the print canine speed is changed.
Thus, there is a need for improved apparatus and methods to overcome these print inconsistencies by maintaining a consistent degree of cure in a single pass of the curing system.